1. Field
The invention relates in general to a capacitance measurement circuit and method for measuring a capacitance of an under-test capacitor.
2. Description of the Related Art
Currently, user control interface is implemented by touch control switch such as capacitive switch. When touched by the user, the touch control switch will respond to the user's control command to execute corresponding operations such as the ON/OFF operations.
To further improve the convenience in use, touch panel or display touch panel (having both display and touch control functions) is already provided. The touch panel or the display touch panel responses to the user's input and clicking etc. The touch panel or the display touch panel is used in various electronic devices such as mobile phones for example. The user can operate the electronic device by clicking on the touch panel or the display touch panel and such operation mode is indeed much more user friendly.
When the user operates a capacitive touch panel, a capacitive display touch panel, or a capacitive switch, the capacitance of the under-test (unknown) capacitor therein will change accordingly. Thus, the user's operation (for example, whether the user presses the switch) or the user's touch position on the touch panel or the display touch panel can be detected. However, how to design a capacitance measurement circuit capable of detecting the under-test capacitor so as to improve the performance of the electronic device is a focus of development in the semiconductor industry.
In measurement of the capacitance of the under-test capacitor, the voltage on a storage capacitor is measured, wherein the capacitance of the storage capacitor is already known. FIG. 1A and FIG. 1B show voltage curves of a storage capacitor. Different under-test capacitors correspond to different voltage curves.
As indicated in FIG. 1A, the voltages (Vdet1, Vdet2, Vdet3) of the storage capacitors corresponding to different under-test capacitors (Cx1, Cx2, Cx3) are measured at time t. Relative capacitance of the under-test capacitor can be estimated according to these voltage values. Based on the relative capacitance, the absolute capacitance of the under-test capacitor can be obtained accordingly.
As indicated in FIG. 1B, the time periods t1, t2, t3 during which the voltages Vdet of the storage capacitors boosts to a predetermined voltage Vref are measured. Likewise, relative capacitance of the under-test capacitor can be estimated according to these time periods.
When the under-test capacitor has resistor effect (which can be regarded as the under-test capacitor is serially connected to a resistor), the abovementioned voltage curves will change. FIG. 1C and FIG. 1D show voltage of storage capacitors when the under-test capacitor has resistor effect. When the under-test capacitor has resistor effect, it takes a longer period to measure a satisfactory voltage of the storage capacitor. Or, it takes a longer period for the voltage of the storage capacitor to achieve the predetermined voltage Vref. Thus, the speed of capacitance measurement is slowed down.
Therefore, examples of the invention provide a capacitance measurement circuit and a method thereof, capable of shortening the measurement time despite the under-test capacitor has resistor effect.